1. Field of the Invention
The present invention relates to a method of crystallizing aspartame or L-.alpha.-aspartyl-L-phenylalanine methyl ester and the crystalline aspartame afforded by such a method.
2. Discussion of the Background
Aspartame, which is well known is a low-calorie sweetener having a high sweetness, is produced by various methods.
In the industrial production of aspartame, crystallization of aspartame is indispensable in any method for isolating it from a reaction solution to finally obtain the aspartame product. For such crystallization, in general, a method has heretofore been employed in which (i) a crude product containing aspartame is re-dissolved in water, an organic solvent or a water-containing organic solvent; (ii) the resulting solution is cooled by heat-exchange with a coolant (forced circulation indirect cooling system) or by evaporating a part of the solvent under a reduced pressure (self-evaporation system), in a crystallizer equipped with a stirrer so as to precipitate crystals, and (iii) the thus-formed crystals are removed by filtration and dewatering with a centrifugal separator or the like.
However, the aspartame afforded by the method has a fine needle-like and extremely fine cotton whisker-like crystal habit so that the solid-liquid separatability of the aspartame crystals by filtration and dewatering is extremely poor. Because of these effects, the above-mentioned method is seriously flawed for practical purposes.
In addition, in the drying step following the crystallization step, the drying load of the cake of the crystals is naturally high in view of the water content therein, and the bulk specific volume of the dried powder obtained is large so that handling of the powder is extremely difficult.
Further, since such crystals are a fine powder after being dried, they easily scatter, or that is, powdering and scattering of the dried powder from the crystals is noticeable. Moreover, the dried powder with such a fine powdery form has a small dissolution rate as it contains much air in the voids between the fine particles of the powder. Because of these reasons, the powdery aspartame has still further problems with respect to its powdery characteristics.
Therefore, if aspartame crystals having a thick and firm crystal habit could be obtained, they would be extremely advantageous from the view of the process of producing them and the product themselves. As a means of overcoming the above-mentioned problems, a method (static crystallization method) of crystallizing aspartame from its aqueous solution by cooling the-solution has already been proposed, in which an aqueous solution of aspartame is cooled by conductive heat transfer without imparting any forced flow, for example by mechanical stirring, thereto, whereby the aspartame is crystallized as bundle-like crystals where plural needle-like crystals are bundled up to seemingly form one crystals having a thick and firm crystal habit. For a discussion, Japanese Patent Publication No. 2-45638 is referred to.
Thus there remains a need for a method of crystallizing aspartame which is free of the above-described drawbacks. There also remains a need for the crystalline aspartame produced by such a method.